U.S. Pat. No. 3,842,194 of Clemens discloses a video disc having a playback system utilizing variable capacitance. In one configuration of the Clemens system information representative of recorded picture and sound is encoded in the form of a relief pattern in a relatively fine spiral groove on the surface of a disc record. For example, groove widths about 2.7 micrometers and groove depths of about 0.5 micrometer may be used. During playback, a pickup stylus about 2.0 micrometers wide having a thin conductive electrode thereon, for example, about 0.2 micrometer thick, engages the groove as the record is rotated by a supportive turntable. Capacitive variations between the stylus electrode and the record surface are sensed to recover the prerecorded information.
Keizer, in U.S. Pat. No. 4,162,510 entitled "Keel Tipped Stylus for Video Disc Systems" which is incorporated herein by reference, discloses a playback stylus which may be used with the Clemens system. The keel tipped pickup stylus comprises a dielectric support element having a body, a constricted terminal portion and shoulders interconnecting the body with the constricted terminal portion.
The constricted terminal portion is defined by a prow, a substantially flat rear surface remote from said prow, a plurality of substantially parallel side surfaces extending from the side edges of the rear surface, a bottom surface extending from the bottom edge of the rear surface and a pluralty of additional surfaces extending from the prow and intersecting the bottom and the side surfaces. The maximum separation between the substantially parallel side surfaces is less than the given groove width.
Keizer, in U.S. Pat. No. 4,104,832 incorporated herein by reference, discloses a method for manufacturing the keel tipped stylus. To form a keel tipped stylus a tapering support element made from a hard dielectric material, such as diamond or sapphire, is contacted with an abrasive lapping disc having a deep, coarse pitched groove. The lands on the lapping disc lap the shoulders of the keep tipped stylus and the walls of the abrasive groove form the substantially parallel side surfaces of the constricted terminal portion.
Abrasive discs can be generated by a method described in an article by Ryan entitled "Material and Process Development for Video Disc Replication", RCA Review, Vol. 39, pages 87-115 (1978). A master recording is preferably made in photoresist by optical recording techniques, although electron beam or electromechanical recording means may also be employed. A metal matrix master which is a negative replica of the surface of the photoresist master recording is prepared from the photoresist master. A metal matrix mold which is a positive replica of the photoresist master is prepared from the metal matrix master. A metal stamper is then prepared from the metal matrix mold. The metal stamper is a negative of the photoresist master and is used to fabricate plastic discs having the appropriate groove pattern in its surface. The disc is then coated with an abrasive.
Carroll et al in their copending application entitled "Method for Preparing Stylus Lapping Discs", Ser. No. 118,088, filed Feb. 4, 1980, incorporated herein by reference, teach an improved method for preparing keel lapping discs. In their method the junctions in the metal matrix master are filled in to form concave junctions.
Roach et al in their copending application entitled "Method for Preparing Keel Lapping Discs", Ser. No. 128,871 which is being filed concurrently and is incorporated herein by reference, describe an improved method for replicating a spiral groove pattern. In this method, the photoresist master is uniformly irradiated and the irradiated photoresist surface layer is removed. As a result the groove pattern is fully reproduced in the metal matrix master.
Another problem encountered during the replication of the spiral groove pattern is the difficulty in obtaining a uniform, thick coating of photoresist. The thickness in any area is a complex function of a number of factors including spin speed, photoresist composition, temperature and humidity. If the photoresist is applied by spin coating, the resulting photoresist layer may be made axially symmetric, but there is generally considerable variation in photoresist layer thickness as a function of radius. Thus, for example, the photoresist layer may become thinner as the radius decreases. These thickness variations result in lapping discs which are nonuniform and therefore cause difficulties when the stylus lapping process is automated.